1. Field of the Invention
The present invention generally relates to radio mobile communication systems, and in particular to a channel estimation method and system for use in a mobile terminal.
2. Description of the Related Art
In a land mobile communication system, generally, a mobile terminal is subject to frequently changing radio conditions because there are many cases where the mobile terminal moves through an area where many buildings are located or moves rapidly through several cells of the cellular system. Therefore, the mobile terminal is provided with a channel estimation capability so as to switch to another radio channel having better communication quality.
The conventional channel estimation is performed based on bit error rates (BERs) detected from a received signal. More specifically, a channel estimator, receiving the BER for a radio channel now in use, counts the number of times during a constant time period that the BER exceeds a maximum permissible value. If the BER count exceeds a predetermined threshold, it is estimated that the communication quality of the radio channel has been degraded, and a channel changing request is transmitted to the nearest base station.
However, according to the conventional channel estimation method, the BER count frequently exceeds the predetermined threshold when a vehicle is moving through a downtown street. The reason is that fading is periodically caused by multipath propagation among the buildings depending on the speed of the vehicle. Therefore, channel changing requests are frequently transmitted to the base station, which performs the channel change each time a channel changing request is received. In other words, when vehicles are moving in the area where many buildings are located, too many hand-over procedures would be required of base stations. This causes the whole cellular system to reduce in channel use efficiency, resulting in a deteriorated connection reliability.